I'm a guy but I can tell you you're basically screwed if you're a shy quiet kind of a guy as the above two posters have essentially relayed. Sure some shy guys do get girls, quite probably not the ones they wanted just whatever slim pickings are going, so scrapping the bottom of the barrel here, odd few shy guys get lucky of course, but percentagewise this is real small.
Girls basically equate a confident guy with potential, both socially and career wise. Sure he may start of doing some ****e job but so long as he's not a total tosser he could stand a good chance of impressing and moving up the career ladder. I have to admit that even as a straight guy, confident guys come across better visually (not looks wise but in speaking/communicating to others), socially and perhaps charisma/likeability/appeal call it what you will.
Odds are though and I have found this ever time that as the saying goes, 'empty vessels make the most noise' in other words they are all, mouth, mouth, mouth and often know little of any substance or use to anyone. Sometimes the more extreme and perhaps annoying, mouth, mouth, mouth they are the less they know or depth they have, essentially they are just blowhards. Don't underestimate the power of being this though its what often makes up a first impression and many people just see it at face view, 'there confident, hey they MUST know what their talking about - er, no' but many if not all around them are convinced and find them convincing so this is all that counts.
Conversely, if a girl sees you as shy or unconfident even though you may be an expert and excel in what you do she won't see this just the visual of a timid, unconfident and hence weak individual not capable of anything. Once you are sterotyped as something its a bitch to shift the label, so this is one you want to avoid as best you can. Women will equate shyness with low earning potential and ability to get on career wise and will pass you over for someone with a bit of gob, their mistake but one they make time and again, just their preference I guess.